Rangerfinder apparatus

ABSTRACT

In a rangefinder apparatus where the external light luminance is relatively high, the period of each accumulating operation in an integrating circuit, the light-emitting period of an infrared emitting diode (IRED), and the light-emitting interval of the IRED are set to 26 microseconds, 52 microseconds, and 360 microseconds, respectively, and the number of accumulating operations is set to 328. When the external light luminance is relatively low, the accumulating period, the light-emitting period, and the light-emitting interval of the IRED are set to 50 microseconds, 76 microseconds, and 526 microseconds, respectively, and the number of accumulating operations is set to 170; and further the accumulating period, the light-emitting period, and the light-emitting interval are set to 28 microseconds, 54 microseconds, and 374 microseconds, respectively, and the number of accumulating operations is set to 1. Consequently, the respective integration times in both cases become identically 8528 microseconds.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a rangefinder apparatus for measuringthe distance to an object to be measured; and, in particular, to anactive type rangefinder apparatus suitably used in a camera or the like.

2. Related Background Art

In active type rangefinder apparatus used in cameras and the like, aninfrared light-emitting diode (IRED) projects a luminous flux toward anobject to be measured, the reflected light of thus projected luminousflux is received by a position sensitive detector (PSD), a signaloutputted from the PSD is arithmetically processed by a signalprocessing circuit and an arithmetic circuit and then is outputted asdistance information, and the distance to the object is detected by aCPU. In general, since errors may occur when the distance is measuredupon a single light-projecting operation, light is projected a pluralityof times so as to obtain a plurality of distance information items,which are then accumulated at predetermined intervals by an integratingcircuit so as to be integrated and averaged.

In such a rangefinder apparatus, it is preferred that the period of eachaccumulating operation and the number of accumulating operations in theintegrating circuit be set to values corresponding to the external lightluminance. Namely, from the viewpoint of improving the accuracy indistance measurement, it is preferable that the period of eachaccumulating period be elongated when the external light luminance islower than when it is higher.

SUMMARY OF THE INVENTION

However, the period of each accumulating operation set by the CPU islimited to integral multiples of a predetermined time (e.g., 2microseconds) and cannot be set to an arbitrary time. Therefore, if theperiod of each accumulating operation and the number of accumulatingoperations in the integrating circuit are set to values corresponding tothe external light luminance, then the integration time (=period of eachaccumulating operation×number of accumulating operations) may varydepending on the external light luminance. Hence, for accuratelycomputing the measured distance value from the result of integration, itis necessary to carry out the computation in conformity with differentconverting expressions depending on the integration time. In this case,since a plurality of converting expressions are needed, the program inthe CPU increases its size, whereby storage means such as electricallyerasable and programmable read-only memory (EEPROM) and the likenecessitate a larger storage capacity.

In order to overcome the problem mentioned above, it is an object of thepresent invention to provide a rangefinder apparatus which can computethe measured distance value from the result of integration according toa single converting expression even when the period of each accumulatingoperation and the number of accumulating operations in the integratingcircuit are changed.

A first rangefinder apparatus in accordance with the present inventioncomprises: (1) light-projecting means for projecting a luminous fluxtoward an object to be measured; (2) light-receiving means for receivingreflected light of the luminous flux projected to the object at alight-receiving position on a position sensitive detector correspondingto a distance to the object, and outputting a signal corresponding tothe light-receiving position; (3) arithmetic means for carrying out anarithmetic operation according to the signal outputted from thelight-receiving means, so as to output an output ratio signalcorresponding to the distance to the object; (4) integrating means foraccumulating and integrating the output ratio signal, so as to output anintegrated signal corresponding to the result of integration; (5)adjusting means for adjusting a period of each accumulating operationand the number of accumulating operations in the integrating means suchthat an integration time which is the sum of respective periods of theaccumulating operations becomes a constant value; and (6) detectingmeans for detecting the distance to the object according to theintegrated signal outputted from the integrating means.

In this rangefinder apparatus, a luminous flux is outputted from thelight-projecting means toward the object to be measured, and isreflected by the object. The light-receiving means receives thereflected light at a light-receiving position on the position sensitivedetector corresponding to the distance to the object, and outputs asignal corresponding to the light-receiving position. The arithmeticmeans arithmetically operates the signal outputted from thelight-receiving means, and outputs an output ratio signal correspondingto the distance to the object. The integrating means accumulates andintegrates the output ratio signal outputted from the arithmetic means,and outputs an integrated signal corresponding to the result ofintegration. According to the integrated signal outputted from theintegrating means, the detecting means detects the distance to theobject. Here, even when the period of each accumulating operation andthe number of accumulating operations in the integrating means arechanged according to the external light luminance, for example, they areadjusted by the adjusting means such that the integration time, which isthe sum of respective periods of the accumulating operations, becomes aconstant value. As a consequence, the distance to the object is detectedby the detecting means in conformity with a single convertingexpression. In this rangefinder apparatus, it is preferred that, whenadjusting the period of each accumulating operation so as to make itlonger than a predetermined time, the adjusting means adjust the periodof an accumulating operation within a range not shorter than thepredetermined time, such that the integration time in the integratingmeans becomes the above-mentioned constant value.

A second rangefinder apparatus in accordance with the present inventioncomprises: (1) light-projecting means for projecting a luminous fluxtoward an object to be measured; (2) light-receiving means for receivingreflected light of the luminous flux projected to the object at alight-receiving position on a position sensitive detector correspondingto a distance to the object, and outputting a signal corresponding tothe light-receiving position; (3) arithmetic means for carrying out anarithmetic operation according to the signal outputted from thelight-receiving means, so as to output an output ratio signalcorresponding to the distance to the object; (4) integrating means foraccumulating and integrating the output ratio signal, so as to output anintegrated signal corresponding to the result of integration; (5)adjusting means for adjusting a period of each accumulating operationand the number of accumulating operations in the integrating means suchthat an integration time which is the sum of respective periods of theaccumulating operations lies within a constant range including apredetermined value; and (6) detecting means for detecting the distanceto the object according to the integrated signal outputted from theintegrating means in conformity with a converting expression for a casewhere the integration time in the integrating means is at thepredetermined value.

This rangefinder apparatus operates substantially similarly to the firstrangefinder apparatus except for the following points. Namely, in thisrangefinder apparatus, even when the period of each accumulatingoperation and the number of accumulating operations in the integratingmeans are changed according to the external light luminance, forexample, they are adjusted by the adjusting means such that theintegration time, which is the sum of respective periods of theaccumulating operations, lies within a constant range including apredetermined value. Then, for detecting the distance to the object inthe detecting means, a converting expression for the case where theintegration time is at the predetermined value is used. Preferably, inthis rangefinder apparatus, the adjusting means adjusts the integrationtime in the integrating means to one of a plurality of values andemploys an average value of the plurality of values as the predeterminedvalue.

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not to beconsidered as limiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configurational view of the rangefinder apparatus inaccordance with a first embodiment of the present invention;

FIG. 2 is a circuit diagram of the first signal processing circuit andintegrating circuit in the rangefinder apparatus in accordance with thefirst embodiment;

FIG. 3 is a timing chart for explaining operations of the rangefinderapparatus in accordance with the first embodiment;

FIG. 4 is a timing chart for explaining timings of control signals atthe time of first integration in the rangefinder apparatus in accordancewith the first embodiment;

FIGS. 5A to 5C are timing charts for explaining the timings of controlsignals at the time of the first integration in the rangefinderapparatus in accordance with the first embodiment;

FIG. 6 is a graph for explaining a method of computing a distance signalfrom an AF signal which is the result of integration in the rangefinderapparatus in accordance with the first embodiment; and

FIG. 7 is a graph for explaining a method of computing a distance signalfrom an AF signal which is the result of integration in the rangefinderapparatus in accordance with a second embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the present invention will be explainedin detail with reference to the accompanying drawings. Here, in theexplanation of the drawings, constituents identical to each other willbe referred to with letters or numerals identical to each other, withouttheir overlapping descriptions being repeated. Also, the followingexplanation relates to cases where active type rangefinder apparatus inaccordance with these embodiments are employed as a rangefinderapparatus of an autofocus type camera.

First Embodiment

First, the overall configuration of the rangefinder apparatus inaccordance with the first embodiment will be explained. FIG. 1 is aconfigurational view of the rangefinder apparatus in accordance withthis embodiment.

A CPU 1 is used for controlling the whole camera equipped with thisrangefinder apparatus, and controls the whole camera including therangefinder apparatus according to a program and parameters prestored inan EEPROM 2. In the rangefinder apparatus shown in this drawing, the CPU1 regulates a driver 3, so as to control the emission of infrared lightfrom an IRED (infrared light-emitting diode) 4. Also, the CPU 1 controlsactions of an autofocus IC (AFIC) 10, and inputs the AF signal outputtedfrom the AFIC 10. Further, the CPU 1 inputs the value of external lightluminance measured by a photometric sensor 71.

By way of a light-projecting lens 101 disposed at the front face of theIRED 4, the infrared light emitted from the IRED 4 is projected onto theobject to be measured. The infrared light is partly reflected by theobject, and the resulting reflected light is received, by way of alight-receiving lens 102 disposed at the front face of a PSD (positionsensitive detector) 5, at a position on the light-receiving surface ofthe PSD 5. This light-receiving position corresponds to the distance tothe object. Then, the PSD 5 outputs two signals I₁ and I₂ whichcorrespond to the light-receiving position. The signal I₁ is a near-sidesignal which has a greater value as the distance is shorter if thequantity of received light is constant, whereas the signal I₂ is afar-side signal which has a greater value as the distance is longer ifthe quantity of received light is constant. The sum of the signals I₁and I₂ represents the quantity of reflected light received by the PSD 5,whereas the output ratio (I₁/(I₁+I₂)) represents the light-receivingposition on the light-receiving surface of the PSD 5, i.e., the distanceto the object. The near-side signal I₁ is inputted to the PSDN terminalof the AFIC 10, whereas the far-side signal I₂ is inputted to the PSDFterminal of the AFIC 10. In practice, however, depending on externalconditions, there are cases where respective signals in which asteady-state light component I₀ is added to the near-side signal I₁ andfar-side signal I₂ are fed into the AFIC 10.

The AFIC 10 is an integrated circuit (IC) constituted by a first signalprocessing circuit 11, a second signal processing circuit 12, anarithmetic circuit 14, and an integrating circuit 15. The first signalprocessing circuit 11 inputs therein a signal I₁+I₀ outputted from thePSD 5, and eliminates the steady-state light component I₀ therefrom,thereby outputting the near-side signal I₁; whereas the second signalprocessing circuit 12 inputs therein a signal I₂+I₀ outputted from thePSD 5, and eliminates the steady-state light component I₀ therefrom,thereby outputting the far-side signal I₂.

The arithmetic circuit 14 inputs therein the near-side signal I₁outputted from the first signal processing circuit 11 and the far-sidesignal I₂ outputted from the second signal processing circuit 12,calculates the output ratio (I₁/(I₁+I₂)), and outputs an output ratiosignal representing the result thereof. The integrating circuit inputstherein the output ratio signal and, together with an integratingcapacitor 6 connected to the C_(INT) terminal of the AFIC 10, integratesthe output ratio by accumulating it a plurality of times, therebyimproving the S/N ratio. Thus accumulated output ratio, i.e., the resultof integration, is outputted from the S_(OUT) terminal of the AFIC 10 asthe AF signal. The CPU 1 inputs therein the AF signal outputted from theAFIC 10, converts the AF signal into a distance signal by carrying out apredetermined arithmetic operation, and sends out the resulting distancesignal to a lens driving circuit 7. According to this distance signal,the lens driving circuit 7 causes a taking lens 8 to effect a focusingaction.

More specific respective circuit configurations of the first signalprocessing circuit 11 and integrating circuit 15 in the AFIC 10 will nowbe explained. FIG. 2 is a circuit diagram of the first signal processingcircuit 11 and integrating circuit 15 in the rangefinder apparatus inaccordance with this embodiment. Here, the second signal processingcircuit 12 has a circuit configuration similar to that of the firstsignal processing circuit 11.

The first signal processing circuit 11 inputs therein the near-sidesignal I₁ with the steady-state light component I₀ outputted from thePSD 5, eliminates the steady-state light component I₀, and outputs thenear-side signal I₁. The current (I₁+I₀) outputted from thenear-distance-side terminal of the PSD 5 is fed to the “−” inputterminal of an operational amplifier 20 in the first signal processingcircuit 11 by way of the PSDN terminal of the AFIC 10. The outputterminal of the operational amplifier 20 is connected to the baseterminal of a transistor 21, whereas the collector terminal of thetransistor 21 is connected to the base terminal of a transistor 22. Thecollector terminal of the transistor 22 is connected to the “−” inputterminal of an operational amplifier 23 and also to the arithmeticcircuit 14. Further, the cathode terminal of a compression diode 24 isconnected to the collector terminal of the transistor 22, whereas thecathode terminal of a compression diode 25 is connected to the “+” inputterminal of the operational amplifier 23. A first reference power source26 is connected to the respective anode terminals of the compressiondiodes 24 and 25.

Also, a steady-state light eliminating capacitor 27 is externallyattached to the CHF terminal of the AFIC 10, and is connected to thebase terminal of a steady-state light eliminating transistor 28 withinthe first signal processing circuit 11. The steady-state lighteliminating capacitor 27 and the operational amplifier 23 are connectedto each other by way of a switch 29, whose ON/OFF is controlled by theCPU 1. The collector terminal of the steady-state light eliminatingtransistor 28 is connected to the “−” input terminal of the operationalamplifier 20, whereas the emitter terminal of the transistor 28 isgrounded by way of a resistor 30.

The integrating circuit 15 has the following configuration. Theintegrating capacitor 6 externally attached to the C_(INT) terminal ofthe AFIC 10 is connected to the output terminal of the arithmeticcircuit 14 by way of a switch 60, to a constant current source 63 by wayof a switch 62, to the output terminal of an operational amplifier 64 byway of a switch 65, and directly to the “−” input terminal of theoperational amplifier 64, whereas the potential thereof is outputtedfrom the S_(OUT) terminal of the AFIC 10. The switches 60, 62, and 65are controlled by control signals from the CPU 1. Also, a secondreference power source 66 is connected to the “+” input terminal of theoperational amplifier 64.

The outline of operations of thus configured AFIC 10 will now beexplained with reference to FIGS. 1 and 2. When not causing the IRED 4to emit light, the CPU 1 keeps the switch 29 of the first signalprocessing circuit 11 in its ON state. The steady-state light componentI₀ outputted from the PSD 5 at this time is inputted to the first signalprocessing circuit 11, and is amplified as a current by the currentamplifier constituted by the operational amplifier 20 and thetransistors 21 and 22. Thus amplified signal is logarithmicallycompressed by the compression diode 24, so as to be converted into avoltage signal, which is then fed to the “−” input terminal of theoperational amplifier 23. When the signal inputted to the operationalamplifier 20 is higher, the cathode potential of the compression diode24 becomes higher, thus increasing the signal outputted from theoperational amplifier 23, whereby the steady-state light eliminatingcapacitor 27 is charged. As a consequence, a base current is supplied tothe transistor 28, so that a collector current flows into the transistor28, whereby, of the signal I₁ fed into the first signal processingcircuit 11, the signal inputted to the operational amplifier 20decreases. In the state where the operation of this closed loop isstable, all of the signal I₀ inputted to the first signal processingcircuit 11 flows into the transistor 28, whereby the chargecorresponding to the base current at this time is stored in thesteady-state light eliminating capacitor 27.

When the CPU 1 turns OFF the switch 29 while causing the IRED 4 to emitlight, of the signal I₁+I₀ outputted from the PSD 5 at this time, thesteady-state light component I₁ flows as the collector current into thetransistor 28 to which the base potential is applied by the chargestored in the steady-state light eliminating capacitor 27, whereas thenear-side signal I₁ is amplified as a current by the current amplifierconstituted by the operational amplifier 20 and the transistors 21 and22 and then is logarithmically compressed by the compression diode 24,so as to be converted into and outputted as a voltage signal. Namely,from the first signal processing circuit 11, the near-side signal I₁ isoutputted alone after the steady-state light component I₀ is eliminated,and thus outputted near-side signal I₁ is inputted to the arithmeticcircuit 14. From the second signal processing circuit 12, on the otherhand, as with the first signal processing circuit 11, the far-sidesignal I₂ is outputted alone after the steady-state light component I₀is eliminated, and thus outputted far-side signal I₂ is inputted to thearithmetic circuit 14.

The near-side signal I₁ outputted from the first signal processingcircuit 11 and the far-side signal I₂ outputted from the second signalprocessing circuit 12 are inputted to the arithmetic circuit 14, and theoutput ratio (I₁/(I₁+I₂)) is calculated by the arithmetic circuit 14 andis outputted to the integrating circuit 15. While the IRED 4 is emittinga predetermined number of pulses of light, the switch 60 of theintegrating circuit 15 is kept in its ON state, whereas the switches 62and 65 are turned OFF, whereby the output ratio signal outputted fromthe integrating circuit 14 is stored in the integrating capacitor 6.When a predetermined number of pulse light emissions are completed, thenthe switch 60 is turned OFF, whereas the switch 65 is turned ON, wherebythe charge stored in the integrating capacitor 6 is reduced by thecharge having an opposite potential supplied from the output terminal ofthe operational amplifier 64. The CPU 1 monitors the potential of theintegrating capacitor 6, so as to measure the time required forregaining the original potential, and determines the AF signal accordingto thus measured time, thereby determining the distance to the object.

Operations of the rangefinder apparatus in accordance with thisembodiment will now be explained. FIG. 3 is a timing chart forexplaining the operations of the rangefinder apparatus in accordancewith this embodiment.

When the release button of the camera is half-pushed, so as to initiatea distance measuring state, a power source voltage supply is resumed inthe AFIC 10, and the switch 65 is turned ON, whereby the integratingcapacitor 6 is preliminarily charged until it attains a referencevoltage V_(REF). Also, the CPU 1 inputs therein the external lightluminance measured by the photometric sensor 71.

After the completion of preliminary charging, the switch 65 is turnedOFF. After the preliminary charging, the IRED 4 is driven by a lightemission timing signal with a duty cycle outputted from the CPU 1 to thedriver 3, as indicated by the line 205 of FIG. 3, so as to emit infraredlight in a pulsing fashion. Here, the period of each light emission andthe number of light emissions in the IRED 4 are determined by the CPU 1according to the external light luminance. The infrared light emittedfrom the IRED 4 is reflected by the object to be measured, and thusreflected light is received by the PSD 5. The arithmetic circuit 14outputs data of the output ratio I₁/(I₁+I₂) for each light emission, andthe integrating circuit 15 inputs therein these data as a distanceinformation signal. The CPU 1 controls the switch 60 at a timingcorresponding to each pulse light emission of the IRED 4, therebyinputting a negative voltage corresponding to the output ratio into theintegrating capacitor 6.

The integrating capacitor 6 of the integrating circuit 15 inputs thereinthe distance information signal outputted from the arithmetic circuit14, and is discharged by a voltage value corresponding to the value ofthe distance information signal. The discharging period (period ofaccumulation) is determined by the CPU 1 according to the external lightluminance. As indicated by the line 204 of FIG. 3, the voltage of theintegrating capacitor 6 decreases stepwise (first integration) everytime the distance information signal is inputted. While the amount ofvoltage drop for each step is distance information per se, the sum ofamounts of voltage drop obtained by individual pulse emissions of theIRED 4 is employed as distance information in this embodiment.

After the input to the integrating capacitor 6 by a predetermined numberof light emissions is completed, the switch 60 is held in its OFF state,and the switch 62 is turned ON by a signal from the CPU 1. As aconsequence, the integrating capacitor 6 is charged at a predeterminedrate determined by the rating of the constant current source 63 (secondintegration).

During the period of this second integration, the voltage of theintegrating capacitor 6 and the reference voltage V_(REF) are comparedwith each other in terms of magnitude. If it is determined that theycoincide with each other, then the switch 62 is turned OFF, so as tostop charging the integrating capacitor 6. Then, the CPU 1 measures thetime required for the second integration. Since the charging speed dueto the constant current source 63 is constant, the AF signal can bedetermined from the time required for the second integration, wherebythe sum of the distance information signals inputted to the integratingcapacitor 6 upon one distance measuring operation, i.e., the distance tothe object to be measured, can be determined.

Thereafter, when the release button is completely pushed, the CPU 1controls the lens driving circuit 7 according to thus determineddistance, so as to cause the taking lens 8 to carry out an appropriatefocusing action, and further performs exposure by opening the shutter(not depicted). Thus, upon a release operation, a series ofphotographing actions comprising preliminary charging, distancemeasurement (first integration and second integration), focusing, andexposure is carried out. Its subsequent photographing operations aresimilar thereto.

Here, in the first integrating action of the rangefinder apparatus, thelight emission timing in the IRED 4 is controlled by the IRED signaloutputted from the CPU 1. The timing of discharging in the integratingcapacitor 6 (opening/closing of the switch 60) is controlled by the INTsignal outputted from the CPU 1. The timing of storing and holding ofthe steady-state light component in the steady-state light eliminatingcapacitor 27 (opening/closing of the switch 29) is controlled by theHOLD signal outputted from the CPU 1. Therefore, the timings of controlsignals at the time of the first integration will now be explainedschematically. FIG. 4 is a timing chart for explaining the timings ofcontrol signals at the time of the first integration in the rangefinderapparatus in accordance with this embodiment.

Each of the HOLD signal and INT signal shown in this chart is a controlsignal supplied from the CPU 1 to the AFIC 10. The control functions ofthe HOLD signal and INT signal are as follows. From the time when theresetting is cleared (at the falling edge of the RESET signal) until theinitial rising edge of the INT signal, the integrating capacitor 6 ispreliminarily charged with the reference voltage V_(REF). From the firstrising edge of the INT signal after clearing the resetting until thefalling edge thereof, the steady-state light eliminating capacitor 27 ispreliminarily charged. After clearing the resetting, during the periodwhen the HOLD signal is at its HIGH level, the steady-state lightcomponent is held by the steady-state light eliminating capacitor 27.After clearing the resetting, during the period when the HOLD signal isat its HIGH level and the INT signal is also at its HIGH level,accumulation is carried out in the integrating capacitor 6. Also, afterclearing the resetting, during the period when the HOLD signal is at itsLOW level and the INT signal is at its HIGH level, second integration iscarried out. On the other hand, the IRED signal is a control signalsupplied from the CPU 1 to the driver 3, and controls the light emissiontiming of the IRED 4.

As shown in this chart, during the period A when the steady-state lightcomponent is held by the steady-state light eliminating capacitor 27 dueto the HOLD signal, the IRED signal causes the IRED 4 to emit light. Inthe period B when the IRED 4 emits light, after the period C requiredfor the output of each amplifier in the circuits to be stabilized haselapsed, the integrating capacitor 6 is discharged by the period D dueto the INT signal. When the period E has elapsed after the completion oflight emission in the IRED 4, the holding of the steady-state lightcomponent by the steady-state light eliminating capacitor 27 isterminated. The IRED 4 emits light at the interval F. In the firstintegration, the discharging of the integrating capacitor 6(accumulation of output ratio signal) in the period D is carried out aplurality of times.

In this embodiment, the period D of each accumulating operation in theintegrating capacitor 6 is adjusted by the CPU 1 according to theexternal light luminance measured by the photometric sensor 71. At thistime, the period D of each accumulating operation in the integratingcapacitor 6 is adjusted such that the integration time (sum ofrespective periods D of integrating operations) becomes a constantvalue. Here, the light-emitting period B of the IRED 4 includes thenecessary period C in addition to the accumulating period D in theintegrating capacitor 6. Also, in order for the light-emitting intensityof pulses of the IRED 4 to be kept constant, it is necessary for theduty cycle of light emission to be kept constant. As a consequence, thelight-emitting interval F is determined in proportion to thelight-emitting period B.

The timings of control signals at the time of the first integration willnow be explained specifically. FIGS. 5A to 5C are timing charts forexplaining the timings of control signals at the time of the firstintegration in the rangefinder apparatus in accordance with thisembodiment. Each numerical value shown in the charts indicates the pulsewidth or pulse interval of a control signal in microseconds. Here, theperiod C from the starting of light emission in the IRED 4 until thestarting of accumulation of the steady-state light component in theintegrating capacitor 6 is set to 26 microseconds, the period E from thecompletion of accumulation in the integrating capacitor 6 until thecompletion of holding of the steady-state light component in thesteady-state light eliminating capacitor 27 is set to 8 microseconds,and each of them is maintained as a constant value.

FIG. 5A is a timing chart of control signals in the case where theexternal light luminance measured by the photometric sensor 71 isrelatively high. Here, the accumulating period D of the integratingcapacitor 6, the light-emitting period B of the IRED 4, and thelight-emitting interval F of the IRED 4 are set to 26 microseconds, 52microseconds, and 360 microseconds, respectively, whereas the number ofaccumulating operations is set to 328. The integration time T₁ in thiscase is:

T₁=26×328=8528 microseconds, and the distance measurement time is 135136microseconds.

On the other hand, FIG. 5B is a timing chart of control signals in thecase where the external light luminance measured by the photometricsensor 71 is relatively low. Here, the accumulating period D of theintegrating capacitor 6, the light-emitting period B of the IRED 4, andthe light-emitting interval F of the IRED 4 are set to 50 microseconds,76 microseconds, and 526 microseconds, respectively, whereas the numberof accumulating operations under this condition is set to 170. Also, theaccumulating period D of the integrating capacitor 6, the light-emittingperiod B of the IRED 4, and the light-emitting interval F of the IRED 4are set to 28 microseconds, 54 microseconds, and 374 microseconds,respectively, whereas the number of accumulating operations under thiscondition is set to 1. The integration time T₂ in this case is:

T₂=50×170+28×1=8528 microseconds, and the distance measurement time is102768 microseconds. Thus, when the external light luminance isrelatively low, the period D of each accumulating operation iselongated, and the period of one of accumulating operations is adjusted,such that the integration time T₂ coincides with the integration timeT₁.

Also, FIG. 5C is a timing chart of control signals in the case where theexternal light luminance measured by the photometric sensor 71 isrelatively low. Here, the accumulating period D of the integratingcapacitor 6, the light-emitting period B of the IRED 4, and thelight-emitting interval F of the IRED 4 are set to 50 microseconds, 76microseconds, and 526 microseconds, respectively, whereas the number ofaccumulating operations under this condition is set to 154. Also, theaccumulating period D of the integrating capacitor 6, the light-emittingperiod B of the IRED 4, and the light-emitting interval F of the IRED 4are set to 46 microseconds, 72 microseconds, and 499 microseconds,respectively, whereas the number of accumulating operations under thiscondition is set to 18. The integration time T₃ in this case is:

T₃=50×154+46×18=8528 microseconds, and the distance measurement time is102986 microseconds. Thus, when the external light luminance isrelatively low, the integration time T₃ can be made to coincide with theintegration time T₁ by elongating the period D of each accumulatingoperation and adjusting the periods of a plurality of accumulatingoperations as well.

FIG. 6 is a graph for explaining a method of computing a distance signalfrom an AF signal which is the result of integration in the rangefinderapparatus in accordance with this embodiment. In this graph, the solidline indicates the relationship between the distance L to the object tobe measured and the AF signal in each of the cases of FIGS. 5A to 5C,i.e., the case where the integration time is 8528 microseconds. Thebroken line indicates the relationship between the distance L to theobject and the AF signal in the case where the accumulating period D ofthe integrating capacitor 6, the light-emitting period B of the IRED 4,and the light-emitting interval F of the IRED 4 are set to 50microseconds, 76 microseconds, and 526 microseconds, respectively,whereas the number of accumulating operations under this condition isset to 170 in FIG. 5B, i.e., the case where the integration time is 8500microseconds. Further, the chain line indicates the relationship betweenthe distance L to the object and the distance signal.

As shown in this graph, each of the AF signal and distance signal issubstantially linear to the reciprocal of the distance L, whereby thedistance signal can be computed from the AF signal according to a linearconverting expression. If the distance signal is to be computedaccurately from the AF signal, then different converting expressions arenecessary for the computation depending on the integration time. Namely,it is necessary to prepare the converting expression for the case wherethe integration time is 8500 microseconds and the converting expressionfor the case where the integration time is 8528 microseconds, separatelyfrom each other. In the rangefinder apparatus in accordance with thisembodiment, however, even when the period of each accumulating operationand the number of accumulating operations in the first integration areadjusted according to the external light luminance, the integration timeis always kept constant, whereby only one converting expression forconverting the AF signal to the distance signal is required. As aconsequence, the program in the CPU 1 would not increase its size, andthe storage capacity needed for the EEPROM 2 would not enhance.

Second Embodiment

The rangefinder apparatus in accordance with the second embodiment willnow be explained. The configuration and basic operations of therangefinder apparatus in accordance with this embodiment are similar tothose in the first embodiment (FIGS. 1 to 4). While the integration timeis always kept constant when the period of each accumulating operationand the number of accumulating operations in the first integration areadjusted according to the external light luminance in the firstembodiment; the integration time is not always kept constant in thesecond embodiment, namely, it is adjusted so as to lie within a constantrange including a predetermined value, whereby the distance signal iscomputed from the AF signal in conformity with a converting expressionfor the case where the integration time is at the above-mentionedpredetermined value. Here, the predetermined value may be the median ofthe constant range, or the average value of two integration times whenthere are only two integration times.

FIG. 7 is a graph for explaining a method of computing a distance signalfrom an AF signal which is the result of integration in the rangefinderapparatus in accordance with this embodiment. In this graph, the solidline indicates the relationship between the distance L to the object tobe measured and the AF signal in the case of FIG. 5A, i.e., the casewhere the integration time is 8528 microseconds. The broken lineindicates the relationship between the distance L to the object and theAF signal in the case where the accumulating period D of the integratingcapacitor 6, the light-emitting period B of the IRED 4, and thelight-emitting interval F of the IRED 4 are set to 50 microseconds, 76microseconds, and 526 microseconds, respectively, whereas the number ofaccumulating operations under this condition is set to 170 in FIG. 5B,i.e., the case where the integration time is 8500 microseconds. Also,the dotted line between the solid line and the broken line indicates therelationship between the distance L to the object and the AF signal inthe case where the integration time is 8514 microseconds (the averagevalue between 8528 microseconds and 8500 microseconds). Further, thechain line indicates the relationship between the distance L to theobject and the distance signal.

As shown in this graph, in each of the respective cases where theintegration time is 8528 microseconds and 8500 microseconds, thedistance signal is computed from the AF signal in conformity with theconverting expression for the case where the integration time is at theaverage value of 8514 microseconds. Namely, only one convertingexpression is necessary for converting the AF signal to the distancesignal. As a consequence, the program in the CPU 1 would not increaseits size, and the storage capacity needed for the EEPROM 2 would notenhance in this case as well. Here, even when there are three or moreintegration times exist, the distance signal can be computed inconformity with the converting expression for the average value of theseplurality of integration times.

Without being restricted to the above-mentioned embodiments, the presentinvention can be modified in various manners. For example, the presentinvention is also applicable to the case where the charging/dischargingof the integrating circuit is the reverse of that in the above-mentionedembodiments, i.e., the integrating circuit in which a plurality ofcharging operations are carried out in the first integration such thatthe voltage of the integrating capacitor increases stepwise and thenonly one discharging operation is carried out in the second integration.

While the distance to the object is obtained on the basis of the timeneeded in the second integral, it may also be obtained on the basis ofthe result of the A/D conversion of the integral voltage value obtainedby the first integral, namely, the voltage value which is reduced due tothe discharge of integral capacitor or the voltage value which isincreased due to the charge of integral capacitor.

Also, though the above-mentioned embodiments explain the cases where theperiod of each accumulating operation and the number of accumulatingoperations in the integrating circuit are changed according to theexternal light luminance, the present invention is also applicable tothe cases where they are changed according to the temperature, powersource voltage, object reflectivity, and the like.

In accordance with the present invention, as explained in detail in theforegoing, even when the period of each integrating operation and thenumber of accumulating operations in the integrating means are changedaccording to the external light luminance, for example, they areadjusted such that the integration time, which is the sum of respectiveperiods of the accumulating operations, becomes a constant value.Alternatively, the period of each integrating operation and the numberof accumulating operations in the integrating means are adjusted suchthat the integration time lies within a constant range including apredetermined value, and a converting expression for the case where theintegration time is at the predetermined value is used for detecting thedistance to the object to be measured. As a consequence, only oneconverting expression is used for detecting the distance to the objectfrom the result of integration, whereby the program in the CPU would notincrease its size, and the storage capacity needed for storage meanssuch as the EEPROM and the like would not enhance.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedfor inclusion within the scope of the following claims.

What is claimed its:
 1. A rangefinder apparatus comprising:light-projecting means for projecting a luminous flux toward an objectat a distance to be measured; light-detecting means for detectingreflected light of the luminous flux projected toward the object at alight-detecting position on a position sensitive detector correspondingto the distance to the object, and outputting a signal corresponding tosaid light-detecting position; arithmetic means for carrying out anarithmetic operation according to the signal output from saidlight-detecting means, and outputting an output ratio signalcorresponding to the distance to the object; integrating means foraccumulating and integrating the output ratio signal, and outputting anintegrated signal corresponding to integration of the output ratiosignal; adjusting means for adjusting a period of each accumulatingoperation and the number of accumulating operations in said integratingmeans such that an integration time, a sum of respective periods of theaccumulating operations, is a constant value; and detecting means fordetecting the distance to the object according to the integrated signaloutput from said integrating means.
 2. The rangefinder apparatusaccording to claim 1, wherein said light-projecting means is an infraredlight-emitting diode.
 3. The rangefinder apparatus according to claim 1,wherein said light-receiving means outputs a near-side signal and afar-side signal.
 4. The rangefinder apparatus according to claim 1,wherein said arithmetic means and said integrating means are part of asingle autofocus integrated circuit.
 5. The rangefinder apparatusaccording to claim 1, wherein, when said adjusting means adjusts theperiod of each accumulating operation to be longer than a threshold timeperiod, said adjusting means adjusts the period of an accumulatingoperation within a range not shorter than the threshold time period,such that the integration time in said integrating means becomes theconstant value.
 6. A rangefinder apparatus comprising: light-projectingmeans for projecting a luminous flux toward an object at a distance tobe measured; light-detecting means for detecting reflected light of theluminous flux projected toward the object at a light-detecting positionon a position sensitive detector corresponding to the distance to theobject, and outputting a signal corresponding to said light-detectingposition; arithmetic means for carrying out an arithmetic operationaccording to the signal output from said light-detecting means, andoutputting an output ratio signal corresponding to the distance to theobject; integrating means for accumulating and integrating the outputratio signal, and outputting an integrated signal corresponding tointegration of the output ratio signal; adjusting means for adjusting aperiod of each accumulating operation and the number of accumulatingoperations in said integrating means such that an integration time, asum of respective periods of the accumulating operations, lies within arange including a fixed value; and detecting means for detecting thedistance to the object according to the integrated signal output fromsaid integrating means in conformity with a converting expression for acase where the integration time in said integrating means is at thefixed value.
 7. The rangefinder apparatus according to claim 6, whereinsaid light-projecting means is an infrared light-emitting diode.
 8. Therangefinder apparatus according to claim 6, wherein said light-receivingmeans outputs a near-side signal and a far-side signal.
 9. Therangefinder apparatus according to claim 6, wherein said arithmeticmeans and said integrating means are part of a single autofocusintegrated circuit.
 10. The rangefinder apparatus according to claim 6,wherein said adjusting means adjusts the integration time in saidintegrating means to one of a plurality of values and employs an averagevalue of the plurality of values as if the fixed value.